1. Field of the Invention
The present invention relates to an image formation apparatus, such as a copying machine, or a laser beam printer. More particularly, the invention relates to an image formation apparatus provided with developing means for developing, by use of developer, the electrostatic images which are formed on an image bearing member, and also, provided with separation means for separating a transfer material from the image bearing member.
2. Related Background Art
Conventionally, for an image formation apparatus it has been the practice to form electrostatic latent images on the surface of an image bearing member which is charged uniformly by use of primary charging means, with the image exposure given to them corresponding to the image information; to develop the electrostatic images by developing means into the toner images; and to charge a transfer material which is arranged to be closely in contact with the toner images on the surface of the image bearing member, hence transferring the toner images to the transfer material. After that, by use of separation means, the charge applied to the transfer means is partly removed to separate the transfer material from the image bearing member, and then, by use of fixation means, the toner images on the transfer material are fixed. In this way, images are output.
FIG. 7 is a view which schematically shows one example of the conventional image formation apparatus described above. The surface of an image bearing member (photosensitive device) 10 is charged uniformly by a means of the primary charger 11 which is formed by corona charging device. Then, the image exposure 12 is given in accordance with the image information by use of the laser scanner or some other image exposure device (not shown). Thus, the electrostatic latent images are formed on the surface of image bearing member 10 corresponding to the image information. The image exposure 12 is of the type that light is irradiated to the darker (black) portion of the images. The electrostatic latent images formed on the surface of the image bearing member 10 are developed into the toner images by use of the development device 13. In the development device 13, the developer that contains at least toner in it, and with the rotations of an agitator (not shown), a developer bearing member 131 that conveys toner to the surface of the toner image bearing member 10. The toner is then charged to a specific polarity by means of friction charging.
Here, the timing roller 14 and the transfer guide 15 form transfer material conveying means. Then, the transfer material 25, which has been fed by a feeding device (not shown), is conveyed to the surface of the image bearing member 10 to be closely in contact with it by use of the timing roller 14 and the transfer guide 15 in synchronism with the toner images on the surface of the image bearing member 10. After that, the transfer device 16, which is formed by the corona charger, gives to the back face of the transfer material 25 the charge having the reverse polarity of the toner. Thus, by the electrostatic force, the toner images on the surface of the image bearing member 10 are transferred to the transfer material 25.
Subsequent to the transfer process, the transfer material 25 is adsorbed electrostatically to the surface of the image bearing member 10 due to the charge given to the back face thereof. Therefore, by use of the separation device 17, the charge, which is provided in the transfer process, is partly removed to reduce the adsorption between the transfer material 25 and the image bearing member 10 in order to separate the transfer material 25 from the image bearing member 10 by the utilization of the elasticity of the transfer material 25. The separation device 17 is the corona charger that gives the charge having the reverse polarity of the charge given by the transfer device 16 or the one to which AC current is superposed. After a transfer process of the kind, the charge given to the back face of the transfer material is removed to weaken the electrostatic adsorption force between the image bearing member and the transfer material. Then, the transfer material is separated by means of the elasticity of the transfer material. This method is called "the electrostatic separation".
The transfer material 25 thus separated from the image bearing member 10, while holding the toner images, is conveyed to the fixation device 21 by use of the conveying device 20. The toner images on the transfer material 25 are fixed to it by use of the fixation device 21. For the fixation device 21, it is generally practiced to adopt a thermal fixation device which thermally fuses the toner whose main component is resin, and causes it to adhere to the transfer material. The transfer material having the toner images thus transferred to it is exhausted by use of the exhaust roller.
In this respect, if the double-face or multiple transfer should be performed, the transfer material is retained in an intermediate tray in the apparatus, and after that, it is again fed and conveyed to the transfer device for the transfer of the toner images to the transfer material which is separated by use of the separation device 17. It is then exhausted outside the apparatus after the fixation has been made by the fixation device 21.
On the other hand, the cleaning device 18 cleans and removes the slightly remaining toner on the surface of the image bearing member 10 after the toner images have been transferred to the transfer material 25. Then, the electrostatic latent images are removed by means of pre-exposure light 19. In this manner, the next image formation cycle will be performed.
However, there are drawbacks given below as to the conventional art described above. In other words, after the researches and studies made by the inventor hereof, it is found that for the following reasons there are some cases where the stable performance of separation cannot be obtained if the transfer material should be separated by use of the electrostatic separation method subsequent to the transfer process.
Firstly, there is a case where the separation performance becomes unstable due to the difference in the elastic properties (rigidity) of transfer materials. For example, by the changes of moisture content in the transfer material, the elasticity thereof changes accordingly. Therefore, if the image formation apparatus is used under a highly humid environment, the moisture content of the transfer material is increased to make its elasticity smaller. In this case, the separation errors may take place. Also, if the kinds of transfer material differ in the thickness thereof or the like, the separation performance may be affected to vary in some cases.
Secondly, a transfer material exhausted from the fixation device is curled by the application of heat, and if the curling is directed so that it is wound around the image bearing member in case of a multiple transfer, the defective separation or re-transfer may take place in some cases.
Thirdly, the separation capability changes depending on the print ratio of images on the head portion of the transfer material that should be conveyed, and if the print ratio on such portion is low, the defective separation tends to take place. In other words, if there is toner between the transfer material and the image bearing member, a gap may take place to weaken the close contactness between them, hence the separation becoming easier. On the other hand, if there is no toner, the gap between the transfer material and the image bearing member becomes extremely small, thus creating a condition that makes the separation extremely difficult.
Also, the print ratio on the leading end and the trailing end of the transfer material may exert a significant influence on curling. In other words, if an image having a higher print ratio is present on the leading end and trailing end of the first face, the curling becomes greater, and the print ratio of the first face exerts influence on the separation capability of the second face eventually.
Fourthly, there is a case where the separation capability changes greatly even for the same white images depending on the variation of the amount of toner adhesion which may be present slightly on the white background (background portion) of such image (hereinafter, referred to as "fogging toner"). Although the toner amount is small, the fogging toner has a great effect on the electrostatic adsorption to be effectuated between the image bearing member and the transfer material after the transfer process.
For example, if the image formation apparatus is not in use for a long time at night or over the week end, the charge that has been given to the toner in the development device by the friction charging is attenuated. As a result, the development device does not regain sufficient developing characteristics for a while after the image formation apparatus is used again after it has been at rest for a long time. The amount of fogging toner is also made smaller accordingly. In other words, the defective separation may take place more often when the image formation apparatus is used after a long rest.
In order to eliminate the drawbacks described above, there has been adopted conventionally a method whereby the de-electrifying capability of the separation device is made larger to deal with the case that the elasticity of a transfer material is small or the amount of the fogging toner adhesion is small. This conventional method, however, has the drawbacks given below.
If the de-electrifying capability of the separation device is made larger, the electric charge given to the back face of the transfer material (the charge having the reverse polarity of the toner image) is reduced. As a result, the electrostatic adsorption between the transfer material and the surface of the image bearing member is weakened accordingly. Therefore, although the separation of the transfer material becomes possible, the holding power of the charge given to the back face of the transfer material is reduced. Then, there occurs the phenomenon that the toner image once transferred to the transfer material is again attracted and taken away to the electrostatic latent image formed on the surface of the image bearing member when the transfer material is separated from the surface of the image bearing member. Such phenomenon as this is referred to as "re-transfer". If the re-transfer takes place, the density of the output image is made extremely lower, and the image quality is also degraded significantly.